The invention relates to an apparatus for guiding a metal band on a gas cushion with blowing boxes which are disposed one after the other in the longitudinal direction of the band and whose nozzle fields, which extend transversally to the longitudinal direction of the band, comprise hole-type nozzles distributed over the field surface area and slotted nozzles along the edges which extend transversally to the longitudinal direction of the band.
In order to enable the conveyance of a metal band floating on a gas cushion in the longitudinal direction of the band it is known (DE 30 26 132 A) to provide blowing boxes which are disposed one after the other in the longitudinal direction of the band and whose nozzle fields extend transversally to the longitudinal direction of the band and comprise hole-type nozzles which are distributed over the field surface area. The gas flowing through said hole-type nozzles against the metal band form a supporting air cushion which is supported by a gas flow from parallel slotted nozzles which extend transversally to the longitudinal direction of the band and delimit the rectangular field surface forming the hole-type nozzles. Although said slotted nozzles which are parallel to the edge prevent the flowing off of the gas from the hole-type nozzle zone in the longitudinal direction of the band, the flowing off of the gas from the gas cushion transversally to the longitudinal direction of the band cannot be prevented through such slotted nozzles. In order to increase the static pressure of the gas cushion between the nozzle slots delimiting the nozzle field and to thus increase the carrying power it is known (DE 298 13 660 U1) to taper the nozzle fields starting from the longitudinal center towards their longitudinal edges, so that the gas flow will also prevent the flowing off of gas from the gas cushion transversally to the longitudinal direction of the band with the desired effect of an increase in the carrying power. With the nozzle fields which outwardly taper transversally to the longitudinal direction of the band the return flow channels between the blowing boxes expand outwardly, which is linked to a respective reduction in the speed of the gas flow flowing through said return flow channels. This means that the pressure drop dependent on the flow speed decreases in the zone of this gas return flow and therefore the carrying power is increased accordingly. The disadvantageous aspect in the nozzle fields tapering towards the longitudinal edges of the metal band is however that as a result of the thus caused uneven distribution of the hole-type nozzles over the width of the metal band only an even heat treatment of the metal band can thus be achieved through the gas flow.
The invention is thus based on the object of providing an apparatus for guiding a metal band on a gas cushion of the kind mentioned above in such a way that with an even distribution of the hole-type nozzles over the width of the band to be treated it is possible to enable an increase in the carrying power without having to increase the gas volume.
The invention achieves this object in such a way that the slotted nozzles along the edges consist of several nozzle slots which are arranged one after the other parallel to the edge and are inclined inwardly with respect to the edge starting from the respective center of the edge.
Since as a result of these measures a tapering of the nozzle field towards the longitudinal edges becomes effective in the zone of the individual nozzle slots which are inwardly inclined with respect to the edge, a resistance is established against the free flowing off of the gas from the air cushion zone transversally to the longitudinal direction of the band by said narrowing of the nozzle field, which resistance leads to an increase in the carrying power because the static pressure in the zone of the gas cushion is increased by hindering the flowing off of the gas towards the longitudinal edges of the metal band. An additional factor is that an additional possibility for flowing off in the longitudinal direction of the band is created between the successive nozzle slots, namely in a zone close to the blowing box before the individual flows converge through the nozzle slots into a common flow directed against the metal band. This means that a part of the gas from the gas cushion zone can flow off between the inclined nozzle slots to the return flow channels between the blowing boxes with the effect that the flow speed of the gas flow deflected in the zone of the metal band is reduced and thus the pressure drop in the zone of this gas flow can be kept comparably small in the zone of the metal band, which is accompanied by a support of the carrying power. Despite this additional possibility for the gas to flow off in the longitudinal direction of the band, the supporting effect of the gas flow produced by the nozzle slots on the gas cushion of the nozzle field is maintained because the individual gas flows converge by the longitudinal slots towards the metal band. The inwardly inclined longitudinal slots along the edges of the nozzle fields extending transversally to the longitudinal direction of the band thus produce an advantageous increase in the carrying power as a result of a cooperation of the obstruction of the gas flow-off transversally to the longitudinal direction of the band on the one hand and the support of an additional gas flow-off in the longitudinal direction of the band without having to take into account any reductions concerning an even heat transmission between the gas flow and the metal band.
Since the length and inclination of the nozzle slots have an influence on the flow behavior in and transversally to the longitudinal direction of the band, the nozzle field can be adjusted to different conditions by the choice of this parameter. The nozzle slots starting out from the respective center of the edges of the nozzle fields can extend parallel with respect to one another. In order to create further possibilities for adjustments, the inclination of the nozzle slots can increase with growing distance from the respective center of the edge in order to have an influence that increases towards the longitudinal edges of the metal band on the flow-off behavior of the gas cushion transversally to the longitudinal direction of the band.
To ensure that it is not necessary to take into account any uneven heat treatment of the band via the gas flow by the nozzle slots as a result of an overlapping zone of nozzle slots placed one after the other, directly adjacent nozzle slots of a row of slots can end in the zone of a common straight line extending in the longitudinal direction of the band, so that transversally to the longitudinal direction of the band the adjacent nozzle slots will follow each other directly without any mutual overlapping, but mutually offset in the longitudinal direction of the band.